Brain Neurostimulator Electrode Fitting

ABSTRACT

Fitting a brain neurostimulator electrode array comprises positioning at least a first electrode in a desired target structure in a first cerebral hemisphere, and positioning at least a second electrode in a corresponding target structure in a contralateral cerebral hemisphere. Electrical stimuli are applied from the first electrode to the desired target structure. Neural responses observed at the second electrode in response to the electrical stimuli are recorded. The fitting of at least one of the first electrode and second electrode is assessed by reference to the recorded neural responses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Australian Provisional Patent Application No. 2015902021 filed 31 May 2015, which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to neural modulation in the brain, and in particular relates to a method for monitoring activity in the brain arising from stimulation in order to optimise implantation of a deep brain stimulator (DBS) electrode array and/or to optimise post-surgical fitting of an implanted DBS array.

BACKGROUND OF THE INVENTION

Neuromodulation involves applying an electric stimulus to biological tissue in order to produce a therapeutic effect. Neuromodulation can be non-invasive such as by transcutaneous electrical nerve stimulation (TENS), transcranial magnetic stimulation (TMS), or highly invasive when requiring the implantation of one or more electrodes and a controlling stimulator as in the case of deep brain stimulation (DBS). DBS has become the most effective treatment for late stage Parkinson's disease, but is a highly invasive therapy requiring the implantations of two leads deep into subcortical nuclei and connection to one or more pulse generators implanted in the chest. Many DBS electrode target structures have been studied to treat a wide variety of diseases, and the preferred location of the electrode varies depending on the disease that is being treated. In the case of Parkinson's disease, the preferred targets are the internal segment of the globus pallidus (GPi) and the subthalamic nucleus (STN). The GPi has also been targeted for Huntington's disease and Tourette's syndrome, the nucleus accumbens has been targeted for chronic depression and alcohol dependence, and the fornix, hypothalamus and nucleus basalis of Meynert have been targeted for Alzheimer's disease.

Parkinson's disease is a degenerative disorder affecting dopamine-releasing cells in the substantia nigra. Many theories describing the functioning of the basal ganglia and how this degeneration relates to Parkinson's disease have been proposed, however all such theories have significant inadequacies in describing all aspects of Parkinson's disease, and understanding the mechanisms of DBS remains the focus of considerable research effort.

A significant reason for the lack of understanding about the mechanisms of DBS and the basal ganglia is the difficulty of measuring the direct responses of the nervous tissue to stimulation. Most of the findings are based on single-cell measurements on efferent structures and, until recently, it was impossible to adequately measure the direct compound response of the target structures because when recording close to the stimulation site, large artefacts (electrical and electrode artefacts) tend to mask the tissue response.

In this light, implantation of a DBS electrode array typically involves inserting the array into the target structure by stereotaxy, to position or physically fit the electrode array at a position defined by three dimensional coordinates. However, given the anatomical and neurological variations between patients, and the relatively large size of each electrode contact relative to the neural structures of interest, stereotaxy positioning generally will not optimise either therapeutic effect or stimulus power minimisation. Accordingly, after surgical implantation, device fitting will typically further comprise clinical exploration of stimulus parameters by trial and error, and selection of electrodes of the array and stimulation parameters which produce the best therapeutic effect.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

In this specification, a statement that an element may be “at least one of” a list of options is to be understood that the element may be any one of the listed options, or may be any combination of two or more of the listed options.

SUMMARY OF THE INVENTION

According to a first aspect the present invention provides a method of fitting a brain neurostimulator electrode array, the method comprising:

positioning at least a first electrode in a desired target structure in a first cerebral hemisphere;

positioning at least a second electrode in a corresponding target structure in a contralateral cerebral hemisphere;

applying electrical stimuli from the first electrode to the desired target structure;

recording neural responses observed at the second electrode in response to the electrical stimuli; and

assessing the fitting of at least one of the first electrode and second electrode by reference to the recorded neural responses.

According to a second aspect the present invention provides a brain neurostimulator device comprising:

at least a first electrode configured to be positioned in a desired target structure in a first cerebral hemisphere;

at least a second electrode configured to be positioned in a corresponding target structure in a contralateral cerebral hemisphere;

a pulse generator configured to apply electrical stimuli from the first electrode to the desired target structure;

measurement circuitry configured to record neural responses observed at the second electrode in response to the electrical stimuli; and

a processor for assessing the fitting of at least one of the first electrode and second electrode by reference to the recorded neural responses.

The present invention further provides computer software, or a computer program product comprising computer program code means, or a non-transitory computer readable medium, or a computing device operating under the control of said software or product, configured to apply electrical stimuli from a first electrode to a desired target structure in a first cerebral hemisphere, and further configured to receive recorded neural responses observed at a second electrode positioned in a corresponding target structure in a contralateral cerebral hemisphere in response to the electrical stimuli, and to assess the fitting of at least one of the first electrode and second electrode by reference to the recorded neural responses.

The neurostimulator may comprise a deep brain stimulator.

The target structure may comprise the subthalamic nucleus.

Thus, the present invention provides for deep brain electrode fitting to be performed by ipsilateral stimulation and contralateral neural response measurement. Some embodiments of the present invention may fit the first electrode by reference to the recorded neural responses, recognising that observed contralateral neural responses peak when the ipsilateral stimulus electrode implantation reaches the desired location, such as the caudal portion of the STN. Additional or alternative embodiments may fit the second electrode by reference to the recorded neural responses, recognising that the observed contralateral neural responses peak when the contralateral recording electrode implantation reaches the desired location, such as the caudal portion of the STN. Longitudinal and axial positioning can thus be optimised intra-operatively, and for example the ipsilateral and contralateral electrode positioning may be iteratively optimised in an alternating manner. Moreover, electrode fitting may be further optimised in some embodiments by the additional step of reversing the roles of the first and second electrodes so that stimuli are applied from the second electrode and contralateral responses are recorded by the first electrode. Post-operative longitudinal electrode selection and/or axial electrode selection and/or circumferential electrode selection can also be so performed.

The neural measurement is preferably obtained in accordance with the teaching of International Patent Publication No. WO2012/155183 by the present applicant, the content of which is incorporated herein by reference.

By assessing fitting of the first electrode to the target structure some embodiments of the present invention may deliver a diagnostic method. The presence, amplitude, morphology, and/or latency of the contralateral neural response may be compared to healthy ranges and/or monitored for changes over time in order to diagnose a disease state. For example an absence of or abnormal morphology of the contralateral response could indicate an inter-hemispheric neural connectivity problem that may not be symptomatic of Parkinson's disease but could induce similar clinically observable symptoms. The knowledge about the morphology of the contralateral responses provided by the present invention can therefore in some embodiments be used as a diagnostic tool beyond only the targeted disease, and may thus provide the ability to separately identify symptoms which may not be treated by the therapy and the ability to guide selection of a supplement to DBS/levodopa therapy. The method of the invention may be applied in some embodiments in order to determine a therapeutic effect of the stimulation, determine a therapeutic effect of medicine, and/or to monitor disease state. A therapeutic response may subsequently be ordered, requested and/or administered based on the diagnosis.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates an implanted deep brain stimulator;

FIG. 2 is a block diagram of the implanted neurostimulator;

FIG. 3 is a schematic illustrating interaction of the implanted stimulator with brain tissue;

FIGS. 4a-4f show the ipsilateral responses and contralateral responses to stimuli applied on one hemisphere; and

FIGS. 5a-5f show the ipsilateral responses and contralateral responses to stimuli applied on the opposite hemisphere.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates an implanted deep brain stimulator 100. Stimulator 100 comprises an electronics module 110 implanted at a suitable location in the patient's chest, and two electrode assemblies 150, 152 implanted within the brain and connected to the module 110 by a suitable lead. Numerous aspects of operation of implanted neural device 100 are reconfigurable by an external control device (not shown). Moreover, implanted neural device 100 serves a data gathering role, with gathered data being communicated to an external device.

FIG. 2 is a block diagram of the implanted neurostimulator 100. Module 110 contains a battery 112 and a telemetry module 114. In embodiments of the present invention, any suitable type of transcutaneous communication, such as infrared (IR), electromagnetic, capacitive and inductive transfer, may be used by telemetry module 114 to transfer power and/or data between an external device and the electronics module 110.

Module controller 116 has an associated memory 118 storing patient settings 120, control programs 122 and the like. Controller 116 controls a pulse generator 124 to generate stimuli in the form of current pulses in accordance with the patient settings 120 and control programs 122. Electrode selection module 126 switches the generated pulses to the appropriate electrode(s) of electrode arrays 150 and 152, for delivery of the current pulse to the tissue surrounding the selected electrode(s). Measurement circuitry 128 is configured to capture measurements of neural responses sensed at sense electrode(s) of the electrode arrays as selected by electrode selection module 126.

FIG. 3 is a schematic illustrating interaction of the electrode array 150 of implanted stimulator 100 with nerve tissue 180, in this case the subthalamic nucleus, however alternative embodiments may be positioned adjacent any suitable brain structure. Array 152 is not shown in FIG. 3 but operates in an equivalent manner in the contralateral cerebral hemisphere. Electrode selection module 126 selects a stimulation electrode 2 of electrode array 150 to deliver an electrical current pulse to surrounding neural tissue 180, and also selects a return electrode 4 of the array 150 for stimulus current recovery to maintain a zero net charge transfer.

Delivery of an appropriate stimulus to the neural tissue 180 evokes a neural response comprising a compound action potential which will propagate along associated neural pathways both in the ipsilateral and contralateral cerebral hemisphere, for therapeutic purposes.

The device 100 is further configured to sense the existence and intensity of compound action potentials (CAPs) propagating within neural tissue 180, whether such CAPs are evoked by the stimulus from electrodes 2 and 4, or otherwise evoked such as by the contralateral electrodes of array 152. To this end, any electrodes of the array 150 may be selected by the electrode selection module 126 to serve as measurement electrode 6 and measurement reference electrode 8. Signals sensed by the measurement electrodes 6 and 8 are passed to measurement circuitry 128, which for example may operate in accordance with the teachings of International Patent Application Publication No. WO2012155183 by the present applicant, the content of which is incorporated herein by reference.

The present invention recognises that neural responses can be observed on the contralateral cerebral hemisphere to the ipsilateral hemisphere being stimulated. Without intending to be limited by theory, this suggests that the contralateral responses seen in the contralateral STN stem from projections from the most caudal parts of the ipsilateral STN being stimulated into the opposite cerebral hemisphere.

The present invention further recognises that such contralateral response observations can be used to optimise the placement of either or both the leads when the STN is targeted. Accordingly, in this embodiment surgical placement of electrodes 150 and 152 is carried out as follows. First, both leads 150 and 152 are implanted to their approximate location using stereotaxy. Stimulation is then delivered on ipsilateral lead 150 while recording on the contralateral lead 152. Ipsilateral lead is progressively relocated by the surgeon, further stimuli are delivered, and the amplitude of the observed contralateral responses is monitored. The responses observed by lead 152 on the contralateral side to the stimulus reach a maxima when the ipsilateral stimulating electrode 150 is ideally located deep in the STN. The placement of lead 150 is therefore optimised for the stimulating lead by moving the lead 150 and observing the response on the contralateral side via lead 152 to identify a maxima in the contralateral response, at which time the ipsilateral electrode can be considered to be ideally located.

The placement of the contralateral second lead 152 can then be adjusted by reversing the roles of leads 152 and 150 and repeating the above steps. Moreover, the position of either or both electrodes may be refined when that electrode is the recording electrode; when the recording electrode moves to the ideal position the observed responses will be maximised.

FIGS. 4a-4f show the ipsilateral responses and contralateral responses to a given stimulus. In particular, FIGS. 4a and 4b illustrate the ipsilateral responses observed on ipsilateral electrodes E3 and E4 in response to stimuli of varying amplitude delivered on ipsilateral electrodes E1 and E2, wherein electrodes E1-E4 are all carried by lead 150, E1 being most deeply inserted and E4 least deeply inserted into the ipsilateral STN. FIGS. 4c-4f illustrate the contralateral responses observed on electrodes E5 to E8 carried by contralateral lead 152, E5 being the most deeply inserted and E8 the least deeply inserted into the contralateral STN.

It is notable that in FIGS. 4c-4f the observed responses on the contralateral side occur simultaneously and that no propagation delay occurs between any of E5 to E8. The timing of the contralateral responses also coincides approximately with the timing of the ipsilateral response recorded furthest away from the stimulus, on ipsilateral electrode E4. Without intending to be limited by theory, it might be surmised that the path taken from the stimulus origin at E1/E2 to the furthest electrode E4 on the ipsilateral lead is of approximately the same distance or delay as the neural pathway(s) from E1/E2 to the electrodes E1-E4 on the contralateral side.

While electrodes arrays 150 and 152 remained in place, stimuli were also applied to the opposite hemisphere by array 152 (instead of array 150 as was the case for FIG. 4). FIGS. 5a-5f show the ipsilateral responses and contralateral responses to the stimuli applied on the opposite hemisphere. In particular, FIGS. 5a-5d illustrate the contralateral responses observed on ipsilateral electrodes E1 to E4 of lead 150 in response to stimuli of varying amplitude delivered on ipsilateral electrodes E5 and E6 of lead 152. Again, E1 was most deeply inserted and E4 least deeply inserted into the contralateral STN. FIGS. 5e and 5f illustrate the ipsilateral responses observed on electrodes E7 and E8 carried by lead 152, E5 being the most deeply inserted and E8 the least deeply inserted into the ipsilateral STN.

FIG. 5 shows that the contralateral effects are bidirectional so that the role of stimulating and recording may be alternated from one array to the other. FIGS. 5a-5d show that the contralateral responses again grow proportionally with stimulus amplitude, and again occur simultaneously. However, the contralateral responses seen in FIGS. 5a-5d exhibit a multi-peak form not seen in FIGS. 4c-4f The peaks of the contralateral responses also do not align with peaks of the ipsilateral ECAPs. Some aspects of the contralateral responses are thus one-sided. It is noted that the patient from whom the results of FIGS. 4 and 5 were obtained exhibited one-sided Parkinsonian rigidity. Moreover, it is noted that the difference in contralateral responses is likely due to different conduction velocities in each hemisphere. On the side where conduction velocity is lowest, as measured by ipsilateral stimulation, the contralateral responses are also slower to arrive which appears to explain why the second peak is visible only in FIGS. 5a -5 d.

There are several degrees of freedom available with respect to the electrode position in the brain and the targeting of the STN. These include the depth of the electrode array, the position of the electrode along the array (i.e. 1 to 4 in a conventional stimulation electrode), the orientation of the electrode, or choice of electrode from a radially distributed electrode array, and the position of the electrode array with respect to the medial-lateral and dorsal-ventral axes of the STN. Any or all such factors may be optimised or at least improved or monitored in accordance with the present invention.

After implantation is complete, clinical fitting can also be conducted in accordance with the present invention. The goal of DBS program parameter and electrode selection is to activate the area efficiently which produces the most robust therapeutic effect. The most caudal part of the STN has been identified to be related to motor functions and is therefore often targeted by DBS. The adjustment of the program parameters as well as the electrode placement can therefore be done by looking at the contralateral response in supplement to the current techniques. Maximising the contralateral response observed in the STN corresponds to a maximised stimulation of the ipsilateral caudal section of the STN.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not limiting or restrictive. 

1. A method of fitting a brain neurostimulator electrode array, the method comprising: positioning at least a first electrode in a desired target structure in a first cerebral hemisphere; positioning at least a second electrode in a corresponding target structure in a contralateral cerebral hemisphere; applying electrical stimuli from the first electrode to the desired target structure; recording neural responses observed at the second electrode in response to the electrical stimuli; and assessing the fitting of at least one of the first electrode and second electrode by reference to the recorded neural responses.
 2. The method of claim 1 wherein the neurostimulator comprises a deep brain stimulator.
 3. The method of claim 2 wherein the target structure comprises a subthalamic nucleus.
 4. The method of claim 1 wherein the first electrode is fitted by reference to the recorded neural responses by seeking a maxima in the recorded neural responses.
 5. The method of claim 1 wherein the second electrode is fitted by reference to the recorded neural responses by seeking a maxima in the recorded neural responses.
 6. The method of claim 1 wherein the fitting comprises intra-operative electrode positioning.
 7. The method of claim 1 further comprising: applying further electrical stimuli from the second electrode to the corresponding target structure in the contralateral cerebral hemisphere; recording, with the first electrode, further contralateral responses from the target structure; and assessing the fitting of at least one of the first electrode and second electrode by reference to the recorded further contralateral responses.
 8. The method of claim 1 wherein the fitting comprises post-operative electrode selection.
 9. The method of claim 1 wherein assessing the fitting comprises comparing a current fitting to a past fitting so as to monitor the fitting for changes over time.
 10. A brain neurostimulator device comprising: at least a first electrode configured to be positioned in a desired target structure in a first cerebral hemisphere; at least a second electrode configured to be positioned in a corresponding target structure in a contralateral cerebral hemisphere; a pulse generator configured to apply electrical stimuli from the first electrode to the desired target structure; measurement circuitry configured to record neural responses observed at the second electrode in response to the electrical stimuli; and a processor for assessing the fitting of at least one of the first electrode and second electrode by reference to the recorded neural responses.
 11. The brain neurostimulator device of claim 10 wherein the neurostimulator comprises a deep brain stimulator.
 12. The brain neurostimulator device of claim 11 wherein the electrodes are configured to be positioned in the subthalamic nucleus.
 13. The brain neurostimulator device of claim 10 wherein the processor is configured to fit the first electrode by seeking a maxima in the recorded neural responses.
 14. The brain neurostimulator device of claim 10 wherein the processor is configured to fit the second electrode by seeking a maxima in the recorded neural responses.
 15. The brain neurostimulator device of claim 10 wherein the processor is configured to provide intra-operative electrode positioning.
 16. The brain neurostimulator device of claim 10 wherein the processor is further configured to: apply further electrical stimuli from the second electrode to the corresponding target structure in the contralateral cerebral hemisphere; record, with the first electrode, further contralateral responses from the target structure; and assess the fitting of at least one of the first electrode and second electrode by reference to the recorded further contralateral responses.
 17. The brain neurostimulator device of claim 10 wherein the processor is configured to provide post-operative electrode selection.
 18. The brain neurostimulator device of claim 10 wherein the processor is configured to assess the fitting by comparing a current fitting to a past fitting so as to monitor the fitting for changes over time.
 19. A non-transitory computer readable medium for fitting a brain neurostimulator electrode array, the computer readable medium comprising computer program code means for applying electrical stimuli from a first electrode positioned in a desired target structure in a first cerebral hemisphere, to the desired target structure; computer program code means for recording neural responses observed at a second electrode in a corresponding target structure in a contralateral cerebral hemisphere in response to the electrical stimuli; and computer program code means for assessing the fitting of at least one of the first electrode and second electrode by reference to the recorded neural responses. 